Coreless multi-layer circuit substrate with minimized pad capacitance

ABSTRACT

A multi layer interconnecting substrate has at least two spaced apart metal layers with a conductive pad on each one of the metal layers. Two different types of insulating layers are placed between the metal layers. The placement is such that one of the two different types of insulating layers is placed between the conductive pads and the other type of insulating layer is placed between the two spaced apart metal layers.

BACKGROUND

The present invention relates to semiconductor devices, in general, andin particular to multilayer resin substrates that are used tointerconnect the semiconductor devices to an external system such as aprinted circuit board (PCB) or the like.

The majority of original equipment manufacturers (OEMs) use third partyelectronic devices in their electrical products. The electrical devicesmay be off the shelf components or custom made. A typical third partyelectrical device usually consists of an integrated circuit (IC) chipoperatively connected to a substrate which is used to connect theelectrical device to the OEM products. The typical substrate is astacked structure consisting of a plurality of resinous layers connectedto a metallic core. Electrical conductors are fabricated on respectivelayers and within vias that interconnect the layers. As a consequence,there are a plurality of communication paths that allow the transmissionof electrical signals between the IC chip and the system to which it isconnected.

The packaging technology has been successful in improving thetransmission characteristics of the substrate by shrinking the thicknessof the core. As a consequence, the substrate structure has evolved froma thick core to a thin core and finally to no core. The no corestructure is often referred to as a coreless structure which has severalattractive features and may trump the use of other designs as soon as itis fully accepted within the electronic packaging industry.

SUMMARY OF THE INVENTION

An embodiment of the present disclosure describes an interconnectingsystem termed a substrate that includes a coreless multi layer stackmounted on a base. The base includes a first (lower) metallic layer anda second (upper) metallic layer. The metallic layers are arranged inspaced relation with a first insulating layer interpose between them. Aball grid area (BGA) pad in the form of a circle is fabricated on thelower of the two metallic layers. A conductive member or pad thatprovides electrical continuity with the BGA pad is fabricated on theupper metallic layer. A second insulating layer is interposed betweenthe BGA pad on the lower metallic layer and the conductive member on theupper metallic layer. The first insulating layer and the secondinsulating layer are in lateral abutment, with each having a differentdielectric constant.

In a second embodiment of the disclosure a semiconductor chip is mountedon the substrate described above.

In another embodiment of the present disclosure an air gap is formedbetween the BGA pad on the lower metallic layer and the conductivemember on the upper metallic layer.

In yet another embodiment of the present disclosure the semiconductorchip and the substrate are mounted on a printed circuit board (PCB),such as a printed circuit board for a computer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A and FIG. 1B depict a side by side plane view of the two bottommetal layers of the interconnecting substrate according to an embodimentof the present disclosure.

FIG. 2 depicts a system including a substrate according to an embodimentof the present disclosure.

FIG. 3 depicts a cross section of the system shown in FIG. 2.

FIG. 4 depicts a diagram of signal traces generated from the simulationof different substrate packages with different pad capacitance. Thediagram demonstrates the improvement made to coreless substrates by thepresent invention.

DESCRIPTION OF EMBODIMENTS

FIG. 1A and FIG. 1B show a plane view of two metallic layers of amultilayer interconnecting substrate according to an embodiment of thepresent disclosure. The two metallic layers are shown in a disassembledside-by-side orientation. When assembled, the metallic layer 100 isplaced next to an external system (not shown) that is connected to themultilayer substrate. This layer is the lowest layer in the multilayersubstrate and is, hereafter referred to as bottom metallic layer 100.The Ball Grid Area (BGA) 102 is fabricated on the bottom metallic layer100. The BGA 102 is conductive and provides electrical communication fora single signal line (not shown) between the multilayer substrate andthe external system. A groove or trench 104 that provides electricalisolation between BGA pad 102 and bottom metallic layer 100 isfabricated around BGA pad 102. As can be seen from the figure other BGApads (shown in partial views) are fabricated on bottom metallic layer100 and serve the same purpose as previously described for BGA pad 102.As a consequence, the partially shown BGA pads will not be discussedfurther.

Still referring to FIG. 1A and FIG. 1B, a plurality of conductive pads,only one of which is labeled 108, are fabricated on metallic layer 106.The conductive pad 108 and others shown but not labeled provideelectrical communication between the upper layers of the substrate andthe BGA pads on bottom metallic layer 100. When assembled, the metalliclayer 106 is placed above and in spaced relation to bottom metalliclayer 100. The alignment between the bottom metallic layer 100 andmetallic layer 106 is such that the conductive pads, such as pad 108, onmetallic layer 106 are in linear alignment with BGA pads, such as BGApad 102, on the bottom metallic lawyer 100.

When the two metallic layers or plates are assembled, as describedherein, bottom metallic layer 100 is the lowest bottom layer andmetallic layer 106 is the next lowest bottom lawyer in the multilayersubstrate stack. When the multi layer substrate stack includes acoreless structure, an excessive pad capacitance is developed betweenpads in the lowest bottom layer and the metal in the next lowest bottomlayer. This excessive pad capacitance adversely affects the quality ofelectrical signals that are propagated through the substrate. Theadverse effect is felt at high frequencies (usually at 4 GHz and above).Most communication system operates in the high frequency range.Therefore, without solving this problem, it is doubtful that thecoreless technology will ever be the dominant packaging technology, eventhough the coreless technology provides a much lower manufacturing costthan the traditional core package technology.

FIG. 2 shows a system 200 including a multi layer substrate or laminatemodule 202 fabricated according to teachings of an embodiment of thepresent disclosure. The terminologies substrate and laminate module areused interchangeably throughout this document. The laminate module 202is coupled by a plurality of BGA pads 204 to a printed circuit board(PCB) 206, such as a mother board for a computer or the like. As statedpreviously, each of the BGA pad is associated with a single signal wireand forms the exchange point between the PCB 206 and the laminate module202 for signals on the associated wire. A semiconductor chip 208 isconnected to the laminate module 202 by a group of C4 solder balls 209.The semiconductor chip 208 includes a silicon die 210 and a cover or lid212 to protect the silicon die that contains the circuits and thetransistors operating to provide functions associated with thesemiconductor chip. For example, if the semiconductor chip is aprocessor it would provide the functions associated with a processor.Likewise, if the chip is a memory chip it would perform memory functionsand so forth. By applying heat to the C4 balls the semiconductor chip208 can be firmly attached to the laminate module 202.

FIG. 3 shows a cross section 300 of the system shown in FIG. 2. In orderto establish connectivity between the FIGS. components that are commonto the FIGS. are identified with the same name but with differentnumerals. As a consequence, silicon die 302 is coupled by C4 solderballs (only one is shown) to Laminate Module 306 which is coupled by BGApads (only one is shown) to Printed Circuit Board (PCB) 308. The silicondie, C4 solder balls, BGA pads, and PCB have already been described andwill not be discussed further in this document.

Still referring to FIG. 3, the substrate or laminate module 306 is aunified structure including a base 310 and a coreless multi layer resinstructure 312 connected to the base. The layers in the coreless multilayer resin structure are substantially the same. Therefore, thedescription of one is intended to cover the others. Each of the layersincludes a non conductive part 314 and a conductive part 316 which isusually located on the top surface of the layer. The non conductive partis at the bottom of the layer and forms an insulating barrier betweenrespective layers of the structure. The non conductive part may befabricated from resin material or other insulating material. Theconductive part of the layers is primarily metal and may be partitionedinto electrical conductors or other pattern as the designer sees fit.

Still referring to FIG. 3, the base 310 includes lower metal layer 318and upper metal layer 320. A BGA pad 322 is fabricated in the lowermetal layer 318. As previously described the BGA pad 322 terminates asingle electrical conductor. A trace of the single conductor will begiven below. A groove or slot 324 is placed around the BGA 322. Thegroove or slot provides electrical isolation between the lower metallayer 318 and BGA pad 322. A conductive pad 326 is fabricated in theupper metal layer 320. The conductive pad 326 and the BGA pad 322 formpart of a conductor that transmits electrical signals through thesystem. As stated above, it has been determined that excessivecapacitance between the pads adversely affect signal quality. To correctthis problem, a first insulator 328 with a first dielectric constant isplaced between BGA pad 322 on the lower metal layer 318 and theconductive pad 326 in upper metal layer 320. A second insulator 330 witha second dielectric constant is placed between the lower metal layer 318and the upper metal layer 320. The first insulator 328 and the secondinsulator 330 are in lateral abutment between the upper metal layer 320and the lower metal layer 318. The dielectric constants for the twoinsulators are different. For example, the dielectric constant for thefirst insulator may be set equal to 2, whereas the dielectric constantfor the second insulator may be set equal to 3.4. It should be notedother appropriate values can be selected by those skilled in the artwithout deviating from the teachings or spirit of the presentdisclosure.

In one embodiment of the present disclosure the first insulator 328 isreplaced by air. This can be achieved by machining a cavity within theupper surface of the BGA pad 322. The cavity is then filled with heatsensitive material that can be dissipated during the manufacturingprocess thereby forming an air gap between the BGA pad 322 andconductive pad 326. Other processes or ways of forming an air gapbetween BGA pad 322 and conductive pad 326 are well within the skills ofone skilled in the art. Therefore, any such implementation of processesor ways is intended to be covered by teachings of the present invention.

Still referring to FIG. 3, an example of a continuous conductor throughthe structure is shown. The conductor begins at pad 344, located in thefirst layer of the structure, and terminates in BGA pad 322. Theintermediate points between the two end points include vias 332, 334,336, 338, 340, 342, and pads 344, 348, 350, 352, and 326. It should benoted that the total height of the stack and the height of each layerwithin the stack are a matter of design choices. Therefore, neither theheight of the stack nor the height of the layers within the stack shouldbe construed as a limitation on the scope of the present invention.

FIG. 4 depicts a graphical representation of simulated signals generatedfrom different simulated core and coreless packages with different padcapacitance. This graphical representation demonstrates the improvementthat the present invention adds to the coreless packaging technology.Some simulations were performed in order to provide reasonablecomparison between the various packages. In particular, a 40 mm packagetrace with 50 ohm characteristic impedance and a capacitivediscontinuity in the middle. The value of this capacitance-discontinuitywas varied such that it would reflect the parallel plate capacitancebetween a circular BGA pad and a reference plane above it. Depending onthe dielectric medium used, the transmission line was simulated usingvarious capacitance-discontinuity values.

Still referring to FIG. 4, insertion losses in decibel (dB) arerepresented by S12 (dB) and is plotted on the vertical axis. Thefrequencies in GHz are plotted on the horizontal axis. The signal trace402 from a traditional core package is used as the standard. The signaltrace 404 represents the trace from a coreless package. The corelesspackage, according to the teachings of an embodiment of the presentdisclosure, with air between the BGA pad on the bottom metal layer andthe conductive pad on the next to bottom metal layer generates thesignal trace 406. The coreless package, according to teachings of thepresent disclosure, with a relatively low dielectric constant, forexample 2 or less, between the BGA pad on the bottom metal layer and theconductive pad on the next to the bottom metal layer generates thesignal trace 408. The closeness of the signal traces to the standardsignal trace 402 the better are the signal characteristics; As aconsequence, the quality of signal characteristics for corelessinterconnecting substrates rank in descending order are provided bytraces 406, 408, and 404.

It is clear from the above discussion that the signal characteristics ofa coreless package are greatly enhanced when the package includes asubstrate that practices the teachings of the present invention.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications are suited to theparticular use contemplated.

1. A multilayer interconnecting substrate comprising: a first metallayer; a BGA pad fabricated in said first metal layer; a second metallayer oppositely placed and in spaced relation to the first metal layer;a conductive pad fabricated in said second metal layer; a firstinsulating layer having a first dielectric constant interpose betweenthe BGA pad and the conductive pad; and a second insulating layer havinga second dielectric constant interpose between the first metal layer andthe second metal layer wherein said second insulating layer is inlateral abutment to the first insulating layer, a value for the firstdielectric constant is approximately 2.0, and a value for the seconddielectric constant is approximately 3.4.
 2. A multilayerinterconnecting substrate comprising: a first metal layer; a BGA padfabricated in said first metal layer; a second metal layer oppositelyplaced and in spaced relation to the first metal layer; a conductive padfabricated in said second metal layer; a first insulating layer having afirst dielectric constant interpose between the BGA pad and theconductive pad; and a second insulating layer having a second dielectricconstant interpose between the first metal layer and the second metallayer wherein said second insulating layer is in lateral abutment to thefirst insulating layer and the first insulating layer includes air.